Supramolecular assemblies of tetravalent terbium complex units: syntheses, structure, and materials properties

There is a growing interest in lanthanide complexes exhibiting unconventional oxidation states, primarily due to their unique electronic structures and accompanying physicochemical properties. Herein, likely the first examples of supramolecular assemblies of non-Ce(iv) tetravalent lanthanide complexes, with the general formula [Tb(OSiPh₃)₄Lx] [1 (n = 2, L1 = 1,2-bis(4-pyridyl)ethane); 2 (L2 = 4,4'-bipyridine), 3 (L3 = 1,2-bis(4-pyridyl)acetylene), 4 (L4 = 1,2-bis(4-pyridyl)ethylene), and 5 (L5 = 1,4-bis(4-pyridyl)benzene)], are reported. Cyclic voltammetry studies show two successive redox events, indicating electronic interactions between the two Tb(iv) centers in the dimeric metallomacrocycle 1. Compounds 2-5 are zig-zag structured coordination polymers featuring complex units of Tb(OSiPh₃)₄ bridged by their respective pyridyl-based ditopic ligands. These tetravalent lanthanide species display impressive stability in air, which is believed to result from the stabilization effect of ligand Lx and the extensive multifarious interactions involving the aromatic rings of the anionic (Ph₃SiO^-) and bridging ligands. UV-vis absorption spectroscopic studies show that 2-5 are semiconducting, each with a narrow bandgap of ca. 1.7 eV. Magnetic property studies yielded magnetic entropy changes of ca. 8.0 J (kg K)^-1 at 2.5 K and 7T, which is reasonable for a complex with high-molecular-weight ligands, suggesting the potential development of Tb(iv) complexes as molecular refrigerants due to their f⁷ electronic configuration.